Internal combustion engine control apparatus and method for controlling the same

ABSTRACT

A forced stoichiometric combustion is executed every time a cumulative travel distance of a vehicle increases by a distance, thereby creating an opportunity to determine an air-fuel ratio value in a region in which a rich spike control is performed. Therefore, the determination of an air-fuel ratio value can be precisely calculated so that the air-fuel ratio value corresponds to a value that reflects a deviation of the actual air-fuel ratio and a proper value. Accordingly, it becomes possible to control, with high precision, correlation between the air-fuel ratio and a proper value based on the determined air-fuel ratio value during a fuel-rich combustion that is caused by the rich spike control.

INCORPORATION BY REFERENCE

[0001] The disclosures of Japanese Patent Application Nos. 2000-156556filed on May 26, 2000 and 2000-335964 filed on Nov. 2, 2000, includingthe specifications, drawings and abstracts are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The invention relates to a control apparatus of an internalcombustion engine and a method for controlling the same.

[0004] 2. Description of Related Art

[0005] Recently, in order to achieve both improved fuel economy andsecured output of an automotive internal combustion engine, a type ofinternal combustion engine has been commercialized in which thecombustion mode is changed between a lean combustion mode and astoichiometric combustion mode based on the state of operation of theengine. An example of such an internal combustion engine is described inJapanese Patent Application Laid-Open No. HEI 7-332071.

[0006] In this type of engine, it is difficult to control oxides ofnitrogen (NOx) by an ordinary three-way catalyst during the leancombustion mode. Therefore, a NOx storage-reduction catalyst is providedin the exhaust system so that NOx produced during the lean combustionmode is absorbed and stored thereby preventing degraded NOx emissions.If the amount of NOx stored in the NOx storage-reduction catalystexceeds an allowable value, a rich spike control is performed thattemporarily shifts the air-fuel ratio to a fuel-rich air-fuel ratio. Dueto a fuel-rich combustion caused by the rich spike control, NOx storedin the NOx storage-reduction catalyst is reduced into nitrogen (N₂) byhydrocarbons (HCs) and the like that exist in exhaust gas, therebypreventing NOx saturation of the catalyst.

[0007] However, if the actual air-fuel ratio varies from a proper valueduring the fuel-rich combustion caused by the rich spike control, thenproper reduction of NOx to N₂ becomes impossible. Furthermore, theamount of HCs emitted can increase, thus leading to deterioration ofexhaust emission. It is therefore conceivable to maintain this propervalue of the air-fuel ratio by controlling the actual air-fuel ratioduring the rich spike control. This control is based on an air-fuelratio value that is determined as a result of a deviation that occursbetween the actual air-fuel ratio and the proper value throughout theair-fuel ratio feedback control during a stoichiometric combustionoperation.

[0008] However, the determination of the air-fuel ratio value isperformed only when a predetermined condition is met during thestoichiometric combustion operation. Therefore, the opportunity todetermine this air-fuel ratio value decreases in an internal combustionengine in which the combustion mode is changed between thestoichiometric combustion mode and the lean combustion mode. Therefore,when the air-fuel ratio is controlled based on the determined air-fuelratio value during the rich spike control, there is no assurance thatthis air-fuel ratio value is a value that corresponds to the deviationbetween the actual air-fuel ratio and the proper value of the ratio. Ifa precise determination of this air-fuel ratio value has not beenaccomplished, good precision in controlling the air-fuel ratio duringthe rich spike control cannot be secured, thereby degrading the exhaustemission.

SUMMARY OF THE INVENTION

[0009] Accordingly, it is an object of the invention to provide acontrol apparatus and a control method of an internal combustion enginein which the combustion mode can be changed, and in which the controlapparatus is capable of accurately controlling the air-fuel ratio in therich spike control in order to curb deterioration of exhaust emission.

[0010] Means for achieving the aforementioned object and operation andadvantages thereof will be described.

[0011] A first mode of the invention is applied to an internalcombustion engine in which a NOx storage-reduction catalyst is providedin an exhaust system, and in which a combustion mode is changed betweena lean combustion and a stoichiometric combustion in accordance with astate of operation of the engine. A control apparatus of the internalcombustion engine in accordance with the first mode executes a richspike control of temporarily shifting an air-fuel ratio to a fuel-richair-fuel ratio when a condition for reducing NOx stored in the NOxstorage-reduction catalyst is met. Furthermore, during the rich spikecontrol, the control apparatus controls the air-fuel ratio based on anair-fuel ratio value that is determined through an air-fuel ratiofeedback control during execution of a stoichiometric combustion,wherein the controller forcibly executes the stoichiometric combustionregardless of the state of operation of the engine, every time theinternal combustion engine is operated for a predetermined period.

[0012] In some cases, the air-fuel ratio varies from a proper value dueto over-time changes in a fuel supply system and an intake system of aninternal combustion engine, and the like. However, in the first mode ofthe invention, this deviation between the actual air-fuel ratio and theproper value is reflected as an air-fuel ratio value through theair-fuel ratio feedback control during the stoichiometric combustionoperation. This construction creates an opportunity to determine theair-fuel ratio value every time the internal combustion engine isoperated for a predetermined time, so that the air-fuel ratio value isprecisely determined as a value that corresponds to the deviationbetween the actual air-fuel ratio and the proper value. As a result,good precision of the air-fuel ratio control based on the determinedair-fuel ratio value during the rich spike control can be secured, and asituation in which the determined air-fuel ratio value becomes animproper value, and therefore the exhaust emission deterioration can beprevented.

[0013] In the first mode of the invention, the controller may forciblyexecute the stoichiometric combustion regardless of the state ofoperation of the engine, every time a cumulative travel distance of avehicle in which the internal combustion engine is installed increasesby a predetermined distance.

[0014] Therefore, every time the cumulative travel distance of thevehicle increases by this predetermined distance, the stoichiometriccombustion is forcibly executed, thereby creating an opportunity todetermine an air-fuel ratio value. Hence, the variation of the actualair-fuel ratio from the proper value due to over-time changes and thelike can be accurately determined and reflected as an air-fuel ratiovalue.

[0015] In the first mode of the invention, the controller may forciblyexecute the stoichiometric combustion regardless of the state ofoperation of the engine, every time a cumulative operation time of theinternal combustion engine increases by a predetermined time.

[0016] Therefore, every time the cumulative operation time of theinternal combustion engine increases by the predetermined time, thestoichiometric combustion is forcibly executed thereby creating anopportunity to determine an air-fuel ratio value. Hence, the variationof the actual air-fuel ratio from the proper value due to over-timechanges and the like can be accurately determined and reflected as anair-fuel ratio value.

[0017] In the above-described mode of the invention, the controller maydiscontinue a forced stoichiometric combustion on a condition that theair-fuel ratio value converges with a predetermined value due to thedetermination performed through the feedback control.

[0018] Therefore, on the condition that the air-fuel ratio convergeswith a predetermined value after the stoichiometric combustion isforcibly executed, the stoichiometric combustion is discontinued, andswitching from the stoichiometric combustion to a lean combustion ispermitted. Hence, during execution of a forced stoichiometric combustionas described above, the air-fuel ratio value is more preciselydetermined to reflect a deviation between the actual air-fuel ratio andthe proper value. Thus, it becomes possible to further improve theprecision of the air-fuel ratio control based on the determined air-fuelratio value during the rich spike control.

[0019] In the above-described mode of the invention, the air-fuel ratiovalue may be determined separately for each one of a plurality ofair-fuel ratio determination regions that are set in an operation regionin which a lean combustion is executed, and the controller maydiscontinue the forced stoichiometric combustion on a condition that allthe determined air-fuel ratio values corresponding to the plurality ofair-fuel ratio determination regions converge.

[0020] Therefore, the air-fuel ratio values that correspond to theair-fuel ratio determination regions can be precisely determined toreflect deviations between the actual air-fuel ratio and the propervalue. Since one of the determined air-fuel ratio values that issuitable to the engine operation state can be used for the air-fuelratio control during the rich spike control, the precision of theair-fuel ratio control can be further improved.

[0021] In the above-described mode of the invention, the controller maydiscontinue the forced stoichiometric combustion regardless ofconvergence of the determined air-fuel ratio values, if an executiontime of the forced stoichiometric combustion equates to at least apredetermined time.

[0022] Therefore, it becomes possible to substantially prevent asituation where the execution time of the forced stoichiometriccombustion becomes excessively long and the fuel economy deteriorates,for example, when the determined air-fuel ratio value does not readilyconverge, or the like.

[0023] The aforementioned predetermined time may be, for example, anupper limit time such that fuel economy deterioration due to thecontinuation of a forced stoichiometric combustion may be acceptable.

[0024] In the above-described mode of the invention, the controller maycontinue the stoichiometric combustion regardless of the execution timeof the stoichiometric combustion until the determined air-fuel ratiovalues converge, if the forced stoichiometric combustion commences whenthe determined air-fuel ratio values equate to an initial value.

[0025] Therefore, if the determined air-fuel ratio value equates to aninitial value for any reason, the determined air-fuel ratio value can beconverged with a predetermined value as soon as possible, and canreflect the deviation between the air-fuel ratio and the proper value.Since the rich spike control is executed with the determined air-fuelratio value equating to the initial value, deterioration of theprecision of the air-fuel ratio control during the rich spike controlcan be curbed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The foregoing and further objects, features and advantages of theinvention will become apparent from the following description ofpreferred embodiments with reference to the accompanying drawings,wherein like numerals are used to represent like elements and wherein:

[0027]FIG. 1 is a schematic diagram illustrating an overall constructionof an engine to which a control apparatus in accordance with a firstembodiment of the invention is applied;

[0028]FIG. 2 is a diagram indicating a region in which a lean combustionoperation is performed, and a region in which a stoichiometriccombustion operation is performed;

[0029]FIG. 3 is a flowchart illustrating a procedure of commandingexecution of a forced stoichiometric combustion operation and aprocedure of permitting a lean combustion operation in accordance withthe first embodiment;

[0030]FIG. 4 is a flowchart illustrating a procedure of commandingexecution of a forced stoichiometric combustion operation and aprocedure of permitting a lean combustion operation in accordance with asecond embodiment;

[0031]FIG. 5 is a flowchart illustrating a procedure of commandingexecution of a forced stoichiometric combustion operation and aprocedure of permitting a lean combustion operation in accordance withthe second embodiment; and

[0032]FIG. 6 is a flowchart illustrating a procedure of determining adetermined air-fuel ratio value in the second embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] A first embodiment in which the invention is applied to anautomotive engine will be described with reference to FIGS. 1 to 3.

[0034] As shown in FIG. 1, a piston 12 of an engine 11 is connected to acrankshaft 14 via a connecting rod 13 so that reciprocating movements ofthe piston 12 are converted into rotation of the crankshaft 14 by theconnecting rod 13. The crankshaft 14 is provided with a signal rotor 14a that has a plurality of protrusions 14 b. Provided at a side of thesignal rotor 14 a is a crank position sensor 14 c that outputs a pulsedsignal corresponding to each protrusion 14 b as the crankshaft 14 turns.

[0035] An intake passage 32 and an exhaust passage 33 are connected to acombustion chamber 16 of the engine 11. A throttle valve 23 foradjusting the amount of air taken into the engine 11 is provided in anupstream portion of the intake passage 32. The degree of opening of thethrottle valve 23 is adjusted based on an amount of depression of anaccelerator pedal 25 (accelerator depression amount) detected by anaccelerator position sensor 26. A vacuum sensor 36 for detecting thepressure in the intake passage 32 (intake pressure) is provided in theintake passage 32 downstream of the throttle valve 23.

[0036] The engine 11 is provided with a fuel injection valve 40 thatdirectly injects fuel into the combustion chamber 16 so as to form amixture of fuel and air. Due to combustion of air-fuel mixture in thecombustion chamber 16, the piston 12 reciprocates, and the crankshaft 14turns, thereby driving the engine 11. A mixture gas existing aftercombustion in the combustion chamber 16 is discharged, as exhaust, outinto the exhaust passage 33.

[0037] The exhaust passage 33 is provided with a NOx storage-reductioncatalyst 33 a that absorbs oxides of nitrogen (NOx) from exhaust gaswhen air-fuel mixture burns at an air-fuel ratio that is on a fuel-leanside of the stoichiometric air-fuel ratio. NOx stored in the NOxstorage-reduction catalyst 33 a is reduced into nitrogen (N₂) byhydrocarbons (HCs) present in exhaust gas when air-fuel mixture burns atan air-fuel ratio that is on a fuel-rich side of the stoichiometricair-fuel ratio. Provided in the exhaust passage 33 upstream of the NOxstorage-reduction catalyst 33 a is an oxygen (O2) sensor 34 that detectsoxygen contained in exhaust gas and outputs a detection signalcorresponding to the concentration of oxygen.

[0038] An electrical construction of a control apparatus of the engine11 in accordance with the embodiment will be described.

[0039] The control apparatus includes an electronic control unit(hereinafter, referred to as “ECU”) 92 for controlling the state ofoperation of the engine 11, such as the mode of combustion, the amountof fuel injected, etc. The ECU 92 has a RAM, that is, a memory fortemporarily storing data inputted from various sensors and the like, abackup RAM, that is, a non-volatile memory for storing data and the likethat need to be retained, during a stop of the engine 11, etc. The ECU92 is connected to the crank position sensor 14 c, the acceleratorposition sensor 26, the oxygen sensor 34, the vacuum sensor 36, the fuelinjection valve 40, etc.

[0040] The ECU 92 switches the mode of combustion of air-fuel mixturebetween a stoichiometric combustion mode in which mixture is burned atthe stoichiometric air-fuel ratio, and a lean combustion mode in whichmixture is burned at an air-fuel ratio on the lean side of thestoichiometric air-fuel ratio, in accordance with the state of operationof the engine. For example, if the state of operation of the engine 11is in a high-speed and high-load region, i.e., stoichiometric combustionregion as indicated by A in FIG. 2, the stoichiometric combustionoperation is performed so as to produce a necessary engine output.

[0041] If the state of operation of the engine 11 is in a low-speed andlow-load region, i.e., lean combustion region as indicated by B in FIG.2, the lean combustion operation is performed to improve the fueleconomy of the engine 11. The combustion mode of the engine 11 is notnecessarily determined in correspondence with the region of operation ofthe engine 11 as mentioned above. For example, if the engine 11 is in anoperation state that is different from a normal state, for example,immediately after the engine 11 is started, the stoichiometriccombustion operation is performed even when the operation state of theengine 11 is in the low-speed and low-load region, i.e., lean combustionregion indicated in by B in FIG. 2.

[0042] During the lean combustion operation of the engine 11, the NOxstorage-reduction catalyst 33 a absorbs NOx from exhaust gas, so thatthe amount of NOx stored in the catalyst 33 a gradually increases. TheECU 92 estimates a current amount of Nox stored based on the state ofengine operation. When the estimated amount of Nox stored exceeds anallowable value, the ECU 92 performs a rich spike control in which theair-fuel ratio of mixture is temporarily shifted to a fuel-rich side ofthe stoichiometric air-fuel ratio e.g., to “12”. Due to the fuel-richmixture combustion caused by the rich spike control, NOx stored in theNOx storage-reduction catalyst 33 a is reduced into N₂ by HCs present inexhaust gas, thereby preventing NOx saturation of the NOxstorage-reduction catalyst 33 a.

[0043] During execution of the rich spike control, the ECU 92 calculatesa final amount of fuel injection Qfin based on equation (1), and drivesand controls the fuel injection valve 40 so that an amount of fuelcorresponding to the final amount of fuel injection Qfin is injectedinto the combustion chamber 16.

Qfin=Qbse*FAF*KG(i)*A  (1)

[0044] Qbse: basic amount of fuel injection

[0045] FAF: feedback correction factor

[0046] KG(i): determined air-fuel ratio value

[0047] A: increasing factor

[0048] The basic amount of fuel injection Qbse, a feedback correctionfactor FAF, a determined air-fuel ratio value KG(i), and an increasingfactor A used to calculate the final amount of fuel injection Qfin willbe described.

[0049] a. Basic Amount of Fuel Injection Qbse

[0050] The basic amount of fuel injection Qbse is a value calculated bythe ECU 92 based on a load rate KL and an engine revolution speed NEsuch that the air-fuel ratio becomes equal to the stoichiometricair-fuel ratio during the stoichiometric combustion operation. The basicamount of fuel injection Qbse increases with increases in the load rateKL under the condition that the engine revolution speed NE is constant.The engine revolution speed NE is detected based on detection signalsfrom the crank position sensor 14 c.

[0051] The load rate KL is a value that indicates the proportion of acurrent load to the maximum engine load of the engine 11. The load rateKL is calculated from the engine revolution speed NE and a parametercorresponding to the amount of air taken into the engine 11. Examples ofthe parameter corresponding to the amount of intake air include anintake pressure PM determined based on a detection signal from thevacuum sensor 36, an accelerator depression amount ACCP determined basedon a detection signal from the accelerator position sensor 26, etc.

[0052] b. Feedback Correction Factor FAF

[0053] The feedback correction factor FAF is a value that is used forcorrecting the amount of fuel injection performed via the ECU 92 so thatthe air-fuel ratio becomes equal to the stoichiometric air-fuel ratioduring the stoichiometric combustion operation. During thestoichiometric combustion operation, the ECU 92 increases or decreasesthe feedback correction factor FAF, with “1.0” being a center value, inaccordance with whether the value indicated by the detection signal fromthe oxygen sensor 34 is on the fuel-rich side or on the fuel-lean sideof the value corresponding to the stoichiometric air-fuel ratio. Thatis, if the value indicated by the detection signal from the oxygensensor 34 is on the rich side of the value corresponding to thestoichiometric air-fuel ratio, the ECU 92 decreases the feedbackcorrection factor FAF to decrease the amount of fuel injected.

[0054] If the value indicated by the detection signal from the oxygensensor 34 is on the lean side of the value corresponding to thestoichiometric air-fuel ratio, the ECU 92 increases the feedbackcorrection factor FAF to increase the amount of fuel injected. Bycorrecting the amount of fuel injected based on the feedback correctionfactor FAF, the air-fuel ratio is brought closer to the stoichiometricair-fuel ratio during the stoichiometric combustion operation. Duringexecution of the rich spike control, the feedback correction factor FAFused to calculate the final amount of fuel injection Qfin in equation(1) is set to “1.0” regardless of the value of the feedback correctionfactor FAF set during the stoichiometric combustion operation.

[0055] c. Determined Air-Fuel Ratio Value KG(i)

[0056] The determined air-fuel ratio value KG(i) is a value that isincreased or decreased, with “1.0” being a center value, based on anaverage value FAFAV of the feedback correction factor FAF during thestoichiometric combustion operation, in such a fashion that the averagevalue FAFAV converges into a predetermined range that contains “1.0”,which is a reference value of the average value FAFAV. During thestoichiometric combustion operation, the ECU 92 increases the determinedair-fuel ratio value KG(i) if the average value FAFAV deviates from thepredetermined range to an increase side. If the average value FAFAVdeviates from the predetermined range to a decrease side, the ECU 92decreases the determined air-fuel ratio value KG(i). By increasing ordecreasing the determined air-fuel ratio value KG(i) based on theaverage value FAFAV in this manner, the average value FAFAV is convergedinto the predetermined range, and the determination of the air-fuelratio value KG(i) is completed. The air-fuel ratio value KG(i) isprovided as a determined value that corresponds to the deviation betweenthe actual air-fuel ratio and the appropriate value.

[0057] The ECU 92 sets a plurality of air-fuel ratio determinationregions i, i.e., i=1, 2, 3, . . . , in accordance with the engine load,i.e., load rate KL, of the engine 11, and sets a determined air-fuelratio value KG(i) for each air-fuel ratio determination region. Of theair-fuel ratio determination regions i, a low load-side air-fuel ratiodetermination region i exists in the low-speed and low-load region,i.e., lean combustion region, indicated by B in FIG. 2. In thisembodiment, the air-fuel ratio determination regions i are set inaccordance with the load rate KL so that only one air-fuel ratiodetermination region i exists in the lean combustion region indicated byB in FIG. 2. Separately for each air-fuel ratio determination region i,the ECU 92 increases or decreases the determined air-fuel ratio valueKG(i) so that the average value FAFAV becomes equal to a value withinthe aforementioned predetermined range, and stores into a predeterminedarea of the backup RAM the determined air-fuel ratio value KG(i) thatbrings the average value FAFAV to a value within the predeterminedrange. For the calculation of a final amount of fuel injection Qfin inequation (1), the ECU 92 uses the determined air-fuel ratio value KG(i)corresponding to the air-fuel ratio determination region i (i=1) presentin the lean combustion region indicated by B in FIG. 2, i.e., determinedair-fuel ratio value KG(1). The reason is because the rich spikecontrol, which uses the final amount of fuel injection Qfin, is executedwhen the state of operation of the engine 11 is in the lean combustionregion indicated by B in FIG. 2.

[0058] d. Increasing Factor A

[0059] The increasing factor A is set to a value that is greater than“1.0” by the ECU 92, and is multiplied by the term “Qbse*FAF*KG(i)”indicated in equation (1). By multiplying the increasing factor A by theterm “Qbse*FAF*KG(i),” the final amount of fuel injection Qfin isincreased so that the air-fuel ratio provided when the rich spikecontrol is executed assumes a value on the rich side of thestoichiometric air-fuel ratio e.g., “12”. Due to the rich combustioncaused by the rich spike control, NOx stored in NOx storage-reductioncatalyst 33 a is reduced into N₂ by HCs in exhaust gas.

[0060] The rich spike control is executed when the operation state ofthe engine 11 is in the low-speed and low-load region indicated by B inFIG. 2, i.e., the lean combustion region. In the lean combustion region,stoichiometric combustion operation is performed only when the engine 11is in an operation state that is different from a normal state, forexample, immediately after the engine 11 is started, or the like.Therefore, the opportunity to execute the stoichiometric combustionoperation decreases, and the opportunity of determining the air-fuelratio value KG(i) (i=1) during the stoichiometric combustion operationalso decreases.

[0061] If the opportunity of determining the air-fuel ratio value KG(i)decreases, the air-fuel ratio value KG(i) may not always correspond tothe deviation between the actual air-fuel ratio and the proper value. Ifthe determined air-fuel ratio value KG(i) is not accurate, the finalamount of fuel injection Qfin calculated from the determined value KG(i)as in equation (1) also reflects an inappropriate value. As a result,during the rich combustion operation caused by the rich spike control,execution of a fuel injection amount control based on the final amountof fuel injection Qfin fails to control the air-fuel ratio to a propervalue e.g., to “12”. Variation between the air-fuel ratio and the propervalue during the rich combustion operation causes NOx stored in the NOxstorage-reduction catalyst 33 a to not be appropriately reduced into N₂,or that the amount of HCs emitted increases, thus leading to degradedexhaust emission.

[0062] In this embodiment, therefore, the stoichiometric combustionoperation is forcibly performed, regardless of the engine operationstate, every time the operation of the engine 11 continues for apredetermined period. For example, every time the cumulative traveldistance of a vehicle in which the engine 11 is installed increases by apredetermined distance, then the stoichiometric combustion operation isforcibly performed. Forced execution of the stoichiometric combustionoperation in this manner creates an opportunity to determine an air-fuelratio value KG(i) (i=1) that corresponds to the air-fuel ratiodetermination region i (i=1) present in the lean combustion region asindicated by B in FIG. 2. As the determination of the air-fuel ratiovalue KG(i) proceeds during the stoichiometric combustion operation inthe lean combustion region, the determined air-fuel ratio value KG(i)converges toward a predetermined value, and thus becomes equal to avalue that corresponds to the deviation between the actual air-fuelratio and the proper value.

[0063] The determination of the air-fuel ratio value KG(i) is moreeasily calculated if the execution period of a forced stoichiometriccombustion operation is longer. However, an excessively elongatedexecution period of the operation becomes disadvantageous with regard tofuel economy of the engine 11. Therefore, this embodiment adopts, as anexecution period of each forced stoichiometric combustion operation, aperiod of time that is needed before the total execution time of thestoichiometric combustion operation in the lean combustion regionindicated by B in FIG. 2 reaches a time t that is suitable to achieveboth the determination of the air-fuel ratio value KG(i) and improvementof fuel economy of the engine 11. An example of the time t may be aminimum time, e.g., 2 or 3 minutes, that is needed before the determinedair-fuel ratio value KG(1) converges as described above.

[0064] As described above, by forcibly executing the stoichiometriccombustion operation every time the engine 11 is operated for thepredetermined period, an opportunity to determine the air-fuel ratiovalue KG(i) in the lean combustion region indicated by B in FIG. 2 iscreated so that the determined air-fuel ratio value KG(i) can correspondto a value that reflects the deviation between the actual air-fuel ratioand the proper value. Then, by calculating a final amount of fuelinjection Qfin from the determined air-fuel ratio value KG(i) as inequation (1), and by performing the fuel injection amount control basedon the final amount of fuel injection Qfin at the time of the richcombustion operation caused by the rich spike control, the air-fuelratio can be prevented from deviating from a proper value, e.g., “12”.Therefore, it is possible to prevent a situation where, due to deviationof the air-fuel ratio from the proper value during the rich combustionoperation, NOx stored in the NOx storage-reduction catalyst 33 a is notproperly reduced into N₂ or the amount of HCs emitted increases therebydeteriorating exhaust emission.

[0065] Next, a procedure of commanding execution of a forcedstoichiometric combustion operation will be described with reference tothe flowchart of FIG. 3 illustrating a stoichiometric combustioncommanding routine. The stoichiometric combustion commanding routine isexecuted via the ECU 92, for example, by a time interrupt of everypredetermined time.

[0066] In the stoichiometric combustion commanding routine, “1” isstored into a predetermined area of the RAM, as a flag F for commandingthe forced stoichiometric combustion operation (S102, S103), when thecumulative travel distance of the vehicle equipped with the engine 11increases by a distance d, e.g., 1000 km, after the previous executionof the forced stoichiometric combustion operation. Furthermore, a valueof a counter C is reset to “0” when F=1 is set and is increased by “1”at every predetermined time on condition that the operation state of theengine 11 is in the lean combustion region indicated by B in FIG. 2 andthe stoichiometric combustion operation is being performed (S105, S106).The counter C indicates a total amount of time of execution of thestoichiometric combustion operation in the lean combustion regionindicated by B in FIG. 2. When the value of the counter C becomes equalto or greater than a predetermined value x, i.e., a value correspondingto the time t, “0” is stored as the flag F into a predetermined area ofthe RAM (S107, S108). If the flag F is “1”, execution of the forcedstoichiometric combustion operation is commanded. If the flag F is “0”,execution of the lean combustion operation is permitted (S109 to S111).

[0067] In step S101, the ECU 92 determines whether the flag F is “0,”i.e., permitting the lean combustion operation. If F=1, i.e., commandingthe stoichiometric combustion operation, the ECU 92 skips steps S102 toS104, and goes to step S105. If F=0, the ECU 92 goes to step S102. Atstep S102, the ECU 92 determines whether the cumulative travel distancehas increased by the distance d, e.g., 1000 km, from the value occurringat the time of the previous execution of the forced stoichiometriccombustion operation. If negative determination is made in step S102,the ECU 92 skips steps S103 to S108, and goes to the step S109. Ifaffirmative determination is made in step S102, the ECU 92 goes to stepS103. The ECU 92 sets the flag F to “1,” i.e., commanding thestoichiometric combustion operation,” in step S103, and resets thecounter C to “0” in step S104. Then, the ECU 92 goes to step S105.

[0068] In step S105, the ECU 92 determines whether the state ofoperation of the engine 11 is within the lean combustion regionindicated by B in FIG. 2, and the flag F is “1,” i.e., commanding thestoichiometric combustion operation, for example, whether all thefollowing conditions (a) to (c) are met.

[0069] (a) the engine revolution speed NE is a value between the rangeof a predetermined value a and a predetermined b;

[0070] (b) the load rate KL is a value between the range of apredetermined value a and a predetermined value β; and

[0071] (c) the flag F is “1,” i.e., commanding the stoichiometriccombustion operation.

[0072] In condition (a), the predetermined value a is set to, forexample, an idle revolution speed, and the predetermined value b is setto, for example, a value that is slightly to a low revolution speed sideof a value that is farthest toward a high revolution speed side withinthe lean combustion region indicated by B in FIG. 2. In condition (b),the predetermined value a is set to the load rate KL occurring duringidle operation, and the predetermined value β is set to a value that isslightly to a low load side of a value that is farthest toward the highload side in the lean combustion region indicated by B in FIG. 2.

[0073] In step S105, if any one of the aforementioned conditions isunmet, step S106 is skipped, and the process proceeds to step S107. Ifall the conditions are met, the process proceeds to step S106. In stepS106, the ECU 92 increases the value of the counter C by “1”. Afterthat, the ECU 92 goes to step S107.

[0074] In step S107, the ECU 92 determines whether the value of thecounter C is at least a predetermined value x, i.e., value correspondingto the time t. If C≧x is not the case, then the ECU 92 skips step S108,and goes to step S109. If C≧x is the case, then the ECU 92 goes to stepS108, in which the ECU 92 sets the flag F to “0.”

[0075] The ECU 92, in step S109, determines whether the flag F is “1”i.e., commanding the stoichiometric combustion operation. If F=1, theECU 92 commands execution of the stoichiometric combustion operation instep S100. If F=0, the ECU 92 permits execution of the lean combustionoperation in step S111. After executing step S110 or step S111, the ECU92 temporarily ends the stoichiometric combustion commanding routine.

[0076] The above-described embodiment achieves the following advantages.

[0077] Every time the cumulative travel distance of the motor vehicleincreases by the distance d, the forced stoichiometric combustionoperation is performed, thereby creating an opportunity to determine anair-fuel ratio value KG(i) in the lean combustion region indicated by Bin FIG. 2, i.e., the region in which the rich spike control isperformed. Since an opportunity of determining the air-fuel ratio valueKG(i) is forcibly created at every predetermined period, it becomespossible to precisely determine the air-fuel ratio value KG(i) so thatthe air-fuel ratio value KG(i) corresponds to a value that reflects thedeviation between the actual air-fuel ratio and the proper value. Byconducting the determination of the air-fuel ratio value KG(i) in thismanner, it becomes possible to control, with high precision, theair-fuel ratio to a proper value, e.g., “12,” based on the determinedair-fuel ratio value KG(i) during the rich combustion operation causedby the rich spike control. Therefore, it is possible to prevent asituation where, due to a deviation between the actual air-fuel ratioand the proper value during the rich combustion operation, NOx stored inthe NOx storage-reduction catalyst 33 a is not properly reduced into N₂or the amount of HCs emitted increases thereby deteriorating exhaustemission.

[0078] Each forced stoichiometric combustion operation is continueduntil the total time of execution of the stoichiometric combustionoperation in the lean combustion region indicated by B in FIG. 2 reachesthe time t, for example, 2 or 3 minutes, that is suitable to achieveboth determination of the air-fuel ratio value KG(i) and improvement offuel economy of the engine 11. Therefore, the determined air-fuel ratiovalue KG(i) is more precisely calculated as a value that corresponds tothe deviation between the actual air-fuel ratio and the proper value, sothat the precision of the air-fuel ratio control based on the determinedair-fuel ratio value KG(i) during the rich spike control can be furtherimproved.

[0079] A second embodiment of the invention will be described withreference to FIGS. 4 and 5. In this embodiment, a plurality of air-fuelratio determination regions i are set in a lean combustion region. Theprocedures of starting and discontinuing a forced stoichiometriccombustion operation for determining an air-fuel ratio value KG(i)separately for each air-fuel ratio determination region i are differentfrom those of the first embodiment.

[0080]FIGS. 4 and 5 show a flowchart illustrating a stoichiometriccombustion commanding routine of this embodiment. This stoichiometriccombustion commanding routine is periodically executed via the ECU 92,as is the case with the routine of the first embodiment (FIG. 3).

[0081] In the stoichiometric combustion commanding routine, steps S201to S204 set a flag F to “1,” i.e., commanding the stoichiometriccombustion operation. The flag F provides a criterion for determiningwhether to command execution of a forced stoichiometric combustionoperation. If the flag F is set to “1,” a forced stoichiometriccombustion operation is started based on the processing described below.

[0082] In steps S201 to S204, it is determined whether the flag F is“0”, for example, whether a forced stoichiometric combustion operationis not being commanded (S201). If the commanding of a stoichiometriccombustion operation (“F=1”) is not present, it is then determinedwhether a forced stoichiometric combustion operation needs to beperformed. This determination is performed based on determination, forexample, as to:

[0083] (a) whether the determined air-fuel ratio values KG(i) i.e., i=1to 5 in this embodiment, that correspond to the air-fuel ratiodetermination regions i, i.e., i=1 to 5 in this embodiment, are aninitial value, e.g., 1.0, (S202); and

[0084] (b) whether the cumulative travel distance of the motor vehiclehas increased by the distance d from the value assumed at the time ofthe previous execution of a forced stoichiometric combustion operation(S203).

[0085] If an affirmative determination is made in either step S202 orstep S203, the flag F is set to “1,” i.e., commanding the stoichiometriccombustion operation (S204). Thus, the forced stoichiometric combustionoperation is started, not only based on the cumulative travel distanceof the motor vehicle, but also when the determined air-fuel ratio valuesKG(i), i.e., i=1 to 5, are 1.0 (initial value). For example, thissituation would apply in a case where the motor vehicle has not beendriven at all, or where the battery has been replaced.

[0086] After the flag F is set to “1” as described above, a counter C1is reset to “0” (S205) that indicates the elapsed time following thecommanding of execution of a forced stoichiometric combustion operation,for example, the time of execution of the stoichiometric combustionoperation. Furthermore, a completion flag X(i) is set to “0”(uncompleted) (S206) for determining whether the determination of anair-fuel ratio value KG(i) that corresponds to the lean combustionregion is completed.

[0087] A plurality of completion flags X(i) are provided that correspondto the determined air-fuel ratio values KG(i), i.e., i=1 to 5, of theair-fuel ratio determination regions i, i.e., i=1 to 5, present in thelean combustion region. The completion flags X(i) that correspond to thedetermined air-fuel ratio values KG(i) are set to “1” (completed), basedon convergence of the determined air-fuel ratio values KG(i), i.e., i=1to 5, to predetermined values.

[0088] It is then determined whether the engine operation state is inthe lean combustion region and whether the flag F1 is “1,” i.e.,commanding the stoichiometric combustion operation, (S207) in FIG. 5. Ifthe determination is affirmative, the determination of the air-fuelratio values KG(i), i.e., i=1 to 5, that correspond to the leancombustion region is performed (S208). Based on convergence of thedetermined air-fuel ratio values KG(i) to predetermined values, thecompletion flags X(i), i.e., i=1 to 5 that correspond to the air-fuelratio determination regions i are set to “1” (completed).

[0089] A determined air-fuel ratio value KG(i) that corresponds to thepresent engine operation state, i.e., load rate KL, is used to calculatea final amount of fuel injection Qfin for the rich spike control.Therefore, the air-fuel ratio control during the rich spike control isperformed by using a determined air-fuel ratio value KG(i) that issuitable for the present engine operation state.

[0090] In the stoichiometric combustion commanding routine, steps S209to S212 are performed to set the flag F to “0,” i.e., permitting a leancombustion operation, so as to discontinue the forced stoichiometriccombustion operation. When the flag F is set to “0,” execution of a leancombustion operation is permitted based on the process described below,and thus the forced stoichiometric combustion is discontinued.

[0091] In steps S209 to S212, the flag F is switched from “1,” i.e.,commanding the stoichiometric combustion operation to “0,” i.e.,permitting the lean combustion operation in a situation as in thefollowing conditions:

[0092] (1) the determination of all the air-fuel ratio values KG(i),i.e., i=1 to 5, is completed; or

[0093] (2) the execution time of the forced stoichiometric combustionreaches or exceeds a predetermined time, and the stoichiometriccombustion operation was not started with the determined air-fuel ratiovalues KG(i), i.e., i=1 to 5, being an initial value “1.0”.

[0094] It is determined whether the present condition is (1) or (2),based on determination, for example, as to:

[0095] (a) whether all the completion flags X(i), i.e., i=1 to 5 are “1”(completed) (S209);

[0096] (b) whether the counter C1 is at least a predetermined value x1(S210); and

[0097] (c) whether the forced stoichiometric combustion operation wasstarted in a state in which the determined air-fuel ratio value KG(i),i.e., i=1 to 5, were the initial value equals 1.0 (S211).

[0098] That is, after affirmative determination is made in step S209,thus determining that the condition (1) is present, or after affirmativedetermination is made in step S210 and negative determination is made instep S211, thus determining that the condition (2) is present, the flagF is set to “0” in step S212.

[0099] Therefore, if the forced stoichiometric combustion operation isstarted based on the determined air-fuel ratio values KG(i), i.e., i=1to 5, being 1.0 (initial value), the flag F is switched from “1” to “0”only after the determination of all the air-fuel ratio values KG(i),i.e., i=1 to 5, is completed, regardless of the execution time of thestoichiometric combustion operation.

[0100] If the stoichiometric combustion operation is started based onthe cumulative travel distance of the motor vehicle increasing by thedistance d following the previous execution of the forced stoichiometriccombustion operation, the flag F is switched from “1” to “0” afterdetermining all the air-fuel ratio values KG(i) (“i=1 to 5”) iscompleted, or after the execution time of the forced stoichiometriccombustion operation reaches or exceeds a predetermined time.

[0101] The execution time of the forced stoichiometric combustionoperation is restricted by the predetermined value x1, corresponding tothe aforementioned predetermined time, used in step S210. Thepredetermined value x1 is preferably a value corresponding to an upperlimit value of the execution time such that the fuel economydeterioration caused by the forced stoichiometric combustion operationis acceptable.

[0102] In the stoichiometric combustion commanding routine, the stepsS213 to S216 command execution of the stoichiometric combustionoperation or permit the lean combustion operation based on the flag F,and periodically increase the value of the counter C1 by “1”. First, itis determined whether the flag F is “1,” i.e., commanding thestoichiometric combustion operation (S213). If the determination isaffirmative, execution of a forced stoichiometric combustion operationis commanded, and the value of the counter C1 indicating the executiontime of the stoichiometric combustion operation is increased by “1”(S214, S215). If negative determination is made in step S213, the forcedstoichiometric combustion operation for conducting the determination ofthe air-fuel ratio values KG(i) is discontinued by permitting executionof a lean combustion operation (S216).

[0103] Next, in step S208 within the stoichiometric combustioncommanding routine, for example, the determination of the air-fuel ratiovalues KG(i), i.e., i=1 to 5 that correspond to the lean combustionregion, will be described with reference to FIG. 6. FIG. 6 is aflowchart illustrating a determination routine for determining eachair-fuel ratio value KG(i). This determination routine is executed viathe ECU 92 every time the step S208 in the stoichiometric combustioncommanding routine (FIG. 5) is reached.

[0104] In the determination routine, it is determined whether conditionsmentioned below are all met (S301).

[0105] (a) a warm-up is not being performed;

[0106] (b) the state of operation of the engine 11 is stable; and

[0107] (c) the air-fuel ratio feedback control is being executed.

[0108] If it is determined that all these conditions are met, it isdetermined which one of the air-fuel ratio determination regions i,i.e., “i=1 to 5,” corresponds to the present engine operation state(S302). The determined air-fuel ratio value KG(i) of the air-fuel ratiodetermining region i is increased or decreased (updated), with 1.0 beinga center value, based on the average value FAFAV of the feedbackcorrection factor FAF so that the feedback correction factor FAFconverges into a predetermined range that contains 1.0, which is areference value of the average value FAFAV (S303).

[0109] It is determined whether the determination of the air-fuel ratiovalue KG(i), i.e., i=1 to 5 is completed, based on a determination as towhether the average value FAFAV of the feedback correction factor FAFhas converged into the predetermined range containing 1.0. Thisdetermination is made based on determination, for example, as to:

[0110] (a) whether the average value FAFAV has continued to be within apredetermined range of 0.95 to 1.05 for at least a predetermined time t1(S304); or

[0111] (b) whether the cumulative time during which the average valueFAFAV is within the predetermined range has reached or exceeded apredetermined time t2 (>t1) (S305).

[0112] The predetermined time t2 is set to a time that is needed beforeit is determined that the average value FAFAV has converged into thepredetermined range in a case where the determination of the air-fuelratio values KG(i) is intermittently performed, for example, a casewhere the air-fuel ratio determination region i for the determiningfrequently changes from one to another, or the like.

[0113] If affirmative determination is made in either step S304 or stepS305, the completion flag X(i) corresponding to the determined air-fuelratio value KG(i) of the air-fuel ratio determination region icalculated in step S302 is set to “1” (completed) (S306).

[0114] In this manner, the determination of the air-fuel ratio valuesKG(i) correspond to the air-fuel ratio determining regions i, i.e., i=1to 5. It is determined whether the determination of an air-fuel ratiovalue KG(i) has been completed, by checking whether the completion flagX(i) that corresponds to that air-fuel ratio value KG(i), i.e., i=1 to5, is “1” (completed). When all the completion flags X(i) (“i=1 to 5”)have been set to “1” (completed), the flag F is set to “0,” i.e.,permitting the lean combustion operation, thus discontinuing the forcedstoichiometric combustion operation.

[0115] The above-described embodiment achieves the aforementionedadvantage of the first embodiment, and also achieves the followingadvantages.

[0116] A plurality of air-fuel ratio determination regions i, i.e., i=1to 5, are set in the lean combustion region, and air-fuel ratio valuesKG(i), i.e., i=1 to 5, are determined separately for the individualair-fuel ratio determination regions i. Therefore, the air-fuel ratiovalues KG(i) corresponding to the air-fuel ratio determination regions ican be precisely calculated to reflect deviations between the actualair-fuel ratio and the proper value. Of the air-fuel ratio values KG(i),i.e., i=1 to 5, a value KG(i) suitable for the engine operation state isused for the air-fuel ratio control during the rich spike control, sothat the precision of the determination of an air-fuel ratio can befurther improved.

[0117] Furthermore, the forced stoichiometric combustion operationstarted based on the cumulative travel distance is discontinued not onlywhen the determination of the air-fuel ratio values KG(i) is completed,but also when the execution time of the stoichiometric combustionoperation reaches or exceeds a predetermined time, for example, when thecounter C1 has reached or exceeded the predetermined value x1.Therefore, the embodiment substantially prevents a situation where theexecution time of the forced stoichiometric combustion operation isexcessively long and the deterioration of fuel economy, for example,when a determined air-fuel ratio value KG(i) does not readily converge.

[0118] In addition, if a forced stoichiometric combustion operation isstarted in a state in which the air-fuel ratio values KG(i), i.e., i=1to 5, are the initial values, i.e., 1.0, for example, in a case wherethe motor vehicle has not been driven at all, or where the battery hasbeen replaced, etc., the stoichiometric combustion operation iscontinued until the determined air-fuel ratio values KG(i), i.e., i=1 to5, converge, regardless of the execution time of the stoichiometriccombustion operation. Therefore, if the determined air-fuel ratio valuesKG(i), i.e., i=1 to 5, are the initial values, it is possible toconverge the determined air-fuel ratio values KG(i) as soon as possibleand determine the values KG(i) to reflect deviations between theair-fuel ratio and the proper value. Hence, it is possible tosubstantially prevent a situation where the rich spike control isexecuted with the determined air-fuel ratio values KG(i), i.e., i=1 to5, being the initial values, therefore deteriorating the precision ofthe air-fuel ratio control during the rich spike control.

[0119] The foregoing embodiments may be modified, for example, in thefollowing manners.

[0120] In the first embodiment, the forced stoichiometric combustionoperation is continued until the total execution time of the forcedstoichiometric combustion operation in the lean combustion regionsindicated by B in FIG. 2 becomes equal to a predetermined time t, e.g.,2 or 3 minutes, that is suitable to achieve both convergence of thedetermined air-fuel ratio value KG(i) to the predetermined value andimprovement in the fuel economy of the engine 11. However, the value ofthe time t may be suitably changed. For example, the time may be set toa value that is longer than 2 or 3 minutes so that the determinedair-fuel ratio value KG(i) is converged to a predetermined value withoutfail during the execution of the forced stoichiometric combustionoperation. Furthermore, the time t may be set to a value that is shorterthan 2 or 3 minutes, so as to further improve the fuel economy of theengine 11.

[0121] In the first embodiment, the forced stoichiometric combustionoperation is ended provided that the total execution time of thestoichiometric combustion operation in the lean combustion regionindicated by B in FIG. 2 reaches or exceeds the time t. However, it isalso possible to adopt a construction in which it is determined whetherthe determined air-fuel ratio value KG(i) has converged to apredetermined value by monitoring the amount of fluctuation of thedetermined air-fuel ratio value KG(i) or the like, and in which theforced stoichiometric combustion operation is ended when it isdetermined that the value KG(i) has converged.

[0122] Although in the first embodiment, the air-fuel ratiodetermination regions i are set so that one air-fuel ratio determinationregion i exists in the lean combustion region indicated by B in FIG. 2,air-fuel ratio determination regions i may also be set so that aplurality of air-fuel ratio determination regions i exist in the leancombustion region.

[0123] In the first embodiment, it is also possible to always execute aforced stoichiometric combustion operation if the cumulative traveldistance is “0.” In this case, even if the cumulative travel distance isreset to “₀,” for example, at the time of replacement of the battery orthe like, a forced stoichiometric combustion operation is executed so asto conduct the determination of air-fuel ratio values KG(i) in the leancombustion region indicated by B in FIG. 2.

[0124] In the second embodiment, the number of air-fuel ratiodetermination regions i in the lean combustion region may be suitablychanged.

[0125] In the second embodiment, the execution time of a forcedstoichiometric combustion operation may also be limited to at most apredetermined time if the forced stoichiometric combustion operation isstarted with the determined air-fuel ratio values KG(i) corresponding tothe lean combustion region being an initial value, as well.

[0126] In the second embodiment, it is not essential to limit theexecution time of a forced stoichiometric combustion operation to atmost a predetermined time.

[0127] Although in the foregoing embodiments, the forced stoichiometriccombustion operation is executed every time the cumulative traveldistance of the motor vehicle increases by the distance d, e.g., 1000km, following the previous execution of the forced stoichiometriccombustion operation, the value of the distance d may be suitablychanged.

[0128] In the foregoing embodiments, it is also possible to execute theforced stoichiometric combustion operation based on the cumulativeoperation time of the engine 11 instead of executing the forcedstoichiometric combustion operation based on the cumulative traveldistance of the motor vehicle. In this case, the forced stoichiometriccombustion operation is executed every time the cumulative operationtime of the engine 11 increases by a predetermined time from the valueoccurring at the time of the previous execution of the forcedstoichiometric combustion operation. Furthermore, it is also possible toexecute the forced stoichiometric combustion operation at everypredetermined period, for example, once a month, a week, or a day, etc.,regardless of the cumulative operation time of the engine 11.

[0129] In the illustrated embodiment, the controllers are implementedwith general purpose processors. It will be appreciated by those skilledin the art that the controllers can be implemented using a singlespecial purpose integrated circuit (e.g., ASIC) having a main or centralprocessor section for overall, system-level control, and separatesections dedicated to performing various different specificcomputations, functions and other processes under control of the centralprocessor section. The controllers can be a plurality of separatededicated or programmable integrated or other electronic circuits ordevices (e.g., hardwired electronic or logic circuits such as discreteelement circuits, or programmable logic devices such as PLDs, PLAs, PALsor the like). The controllers can be suitably programmed for use with ageneral purpose computer, e.g., a microprocessor, microcontroller orother processor device (CPU or MPU), either alone or in conjunction withone or more peripheral (e.g., integrated circuit) data and signalprocessing devices. In general, any device or assembly of devices onwhich a finite state machine capable of implementing the proceduresdescribed herein can be used as the controllers. A distributedprocessing architecture can be used for maximum data/signal processingcapability and speed.

[0130] While the invention has been described with reference to what arepresently considered to be preferred embodiments thereof, it is to beunderstood that the invention is not limited to the disclosedembodiments or constructions. On the contrary, the invention is intendedto cover various modifications and equivalent arrangements.

What is claimed is:
 1. A control apparatus of an internal combustionengine, comprising: a NOx storage-reduction catalyst provided in anexhaust system; and a controller that executes a rich spike control thattemporarily shifts an air-fuel ratio to a fuel-rich air-fuel ratio whena condition for reducing NOx stored in the NOx storage-reductioncatalyst is met, and that, during the rich spike control, controls theair-fuel ratio based on an air-fuel ratio value that is determinedthrough an air-fuel ratio feedback control during execution of astoichiometric combustion, wherein the controller forcibly executes thestoichiometric combustion regardless of a state of operation of theengine, every time the internal combustion engine is operated for apredetermined period.
 2. A control apparatus according to claim 1,wherein the controller forcibly executes the stoichiometric combustionregardless of the state of operation of the engine, every time acumulative travel distance of a vehicle in which the internal combustionengine is installed increases by a predetermined distance.
 3. A controlapparatus according to claim 1, wherein the controller forcibly executesthe stoichiometric combustion regardless of the state of operation ofthe engine, every time a cumulative operation time of the internalcombustion engine increases by a predetermined time.
 4. A controlapparatus according to claim 1, wherein the controller discontinues aforced stoichiometric combustion on a condition that the air-fuel ratiovalue converges with a predetermined value due to the determinationperformed through the feedback control.
 5. A control apparatus accordingto claim 4, wherein: the air-fuel ratio value is determined separatelyfor each one of a plurality of air-fuel ratio determination regions thatare set in an operation region in which a lean combustion is executed;and the controller discontinues the forced stoichiometric combustion ona condition that all the air-fuel ratio values corresponding to theplurality of air-fuel ratio determination regions converge.
 6. A controlapparatus according to claim 4, wherein the controller discontinues theforced stoichiometric combustion regardless of convergence of theair-fuel ratio value, if an execution time of the forced stoichiometriccombustion is at least a predetermined time.
 7. A control apparatus inaccording to claim 6, wherein if the forced stoichiometric combustion isstarted with the air-fuel ratio value being an initial value, thecontroller continues the stoichiometric combustion regardless of theexecution time of the stoichiometric combustion until the air-fuel ratiovalue converges.
 8. A method of controlling an internal combustionengine having a NOx storage-reduction catalyst in an exhaust system,comprising the steps of: executing a rich spike control that temporarilyshifts an air-fuel ratio to a fuel-rich air-fuel ratio when a conditionfor reducing NOx stored in the NOx storage-reduction catalyst is met;and controlling the air-fuel ratio, during the rich spike control, basedon an air-fuel ratio value that is determined through an air-fuel ratiofeedback control during execution of a stoichiometric combustion,wherein the stoichiometric combustion is forcibly executed regardless ofa state of operation of the engine, every time the internal combustionengine is operated for a predetermined period.
 9. The method accordingto claim 8, wherein the stoichiometric combustion is forcibly executedregardless of the state of operation of the engine, every time acumulative travel distance of a vehicle in which the internal combustionengine is installed increases by a predetermined distance.
 10. Themethod according to claim 8, wherein the stoichiometric combustion isforcibly executed regardless of the state of operation of the engine,every time a cumulative operation time of the internal combustion engineincreases by a predetermined time.
 11. The method according to claim 8,wherein the forced stoichiometric combustion is discontinued on acondition that the air-fuel ratio value converges with a predeterminedvalue due to the determination performed through the feedback control.12. The method according to claim 11, wherein: the air-fuel ratio valueis determined separately for each one of a plurality of air-fuel ratiodetermination regions that are set in an operation region in which alean combustion is executed; and the forced stoichiometric combustion isdiscontinued on a condition that all the air-fuel ratio valuescorresponding to the plurality of air-fuel ratio determination regionsconverge.
 13. The method according to claim 11, wherein the forcedstoichiometric combustion is discontinued regardless of convergence ofthe air-fuel ratio values, if an execution time of the forcedstoichiometric combustion is at least a predetermined time.
 14. Themethod according to claim 13, wherein if the forced stoichiometriccombustion is started with the air-fuel ratio values being an initialvalue, the stoichiometric combustion is continued regardless of theexecution time of the stoichiometric combustion until the air-fuel ratiovalues converge.